1. Field of the Invention
The invention pertains generally to the art of high frequency radiometry which in the past has often employed the comparison of the amplitude of a signal to be investigated, such as a thermal or other noise signal, with the amplitude of a locally generated standard noise reference signal. In more particular, the invention pertains to the art of high frequency radiometry in which a single collimating aperture is employed in a novel system for accurately performing both active illuminated and passive radiometric measurements upon objects which represent sources of such thermal or other noise signals.
2. Description of the Prior Art
The comparison radiometer has been used for the study of low-level noise-like radio signals, especially where the amplitudes of those signals are small in comparison to the noise level generated within the radiometer. Comparison radiometers achieve substantial cancellation of the receiver background noise and self-noise, permitting relatively accurate measurements of the magnitudes of low-level radio signals.
One type of radiometer often used in high frequency bands is that in which an incoming signal to be investigated and a standard or calibrated reference noise signal are compared in amplitude. The method consists essentially of the comparison of the unknown noise signal amplitude with a known amplitude noise signal from a calibrated source. In such instruments, the input to receiver elements is switched cyclically between the unknown signal and the noise reference signal at a relatively high rate, and the detected receiver output is coupled to a phase sensing detector operated in synchronism with the switching rate. The amplitude of the final output signal is proportional to the difference between the temperature of the noise reference signal and the effective temperature of the source viewed by the radiometer antenna, because the phase sensing detector acts automatically to substract receiver background and internal noise.
Passive radiometric systems have advantages in that they are passive and therefore do not interfere with operation of other radio equipment; they also operate in most types of weather, are simple and reliable, and are relatively inexpensive. However, the performance of such prior art radiometric systems with respect to targets at relatively great ranges has been less than fully satisfactory because of the limited basic radiometric temperature contrast between such selected targets and their spatial background. Furthermore, there is a significantly increasing dilution effect at large target ranges because of the constant angular width receptivity pattern of directive antennas employed in such radiometer systems.
In more recent radiometric systems, facilities have been added for providing detection of targets at greater ranges and for accurately performing illuminated or active radiometric measurements thereupon as well as passive measurements. In such systems, signals to be investigated are again received by a single antenna and are compared with reference signals provided by a reference signal generator. A switching device cyclically applies the received and reference signals within the intermediate frequency section of the radiometer receiver. The output of the detector of the receiver is an alternating signal having a strong component at the switching frequency. This alternating component is applied through an amplifier to a phase sensitive detector and thence to a signal integrator circuit and may finally operate a display calibrated in terms of temperature.
Such radiometers may be operated in a generally conventional manner as in a passive radiometric system or may be used in a second mode providing transmission of noise signals for illumination of a remote target. The broad band noise power is continuously transmitted, being directed by the single antenna to the selected target. The transmitter and receiver elements cooperate with the single directive antenna, the high frequency portion of the radiometer system being supplied with a suitable signal generation and isolation system for permitting continuous wave transmission when the system is operated in its active mode. Such systems overcome to a useful extent certain fundamental defects of prior art comparison radiometers, including the limited temperature contrast of the usual target with respect to its spatial background, as well as the dilution effect characteristic of conventional radiometric antennas.
More recently, there has been advanced a system additionally lending target range measurement capability to such prior art radiometric systems in a fully compatible manner in such a way that the beneficial features of those prior systems are not compromised. Such a system makes maximum use of common components of existing active-passive systems, uses a single antenna aperture, and uses a single high-frequency oscillator both as transmitter and receiver local oscillator. It additionally retains in active mode operation the beneficial measurement features provided for use in the passive mode of operation. This recent system will be further mentioned herein and provides means for detecting targets at relatively great distances and for accurately performing active illuminated and passive radiometric measurement thereupon, including target range measurements when operating in its active mode. The system features operation selectively in passive or active modes, providing in the active mode incoherent f.m. continuous wave illumination of a selected target for improving the measured radiometric temperature contrast between the target and its spatial background. Composite noise and triangular wave frequency modulation elements in a closed-loop control system yield a constant receiver beat frequency in a sensor system in which range data is derived. The latter prior art system is one in which a composite noise and triangular wave frequency modulation are provided for the high frequency signal illuminating the selected target through a single antenna, thus augmenting the apparent temperature of the selected target with respect to its spatial background and providing a closed-loop control system for extracting target range data.